Solid-state organic light-emitting diode (OLED) displays are of great interest as a superior flat-panel display technology. These displays utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Different organic materials emit different colors of light. However, as the display is used, the organic materials in the display age and become less efficient at emitting light. This reduces the lifetime of the display. The differing organic materials may age at different rates, causing differential color aging and a display whose white point varies as the display is used.
The characteristics of a solid-state display are affected not only by its inherent technology and by the manufacturing processes and materials used to create it, but also by the way in which it is operated. The voltages supplied to the device, current available, the timing of various signal lines, the temperature of operation, etc. all affect the display characteristics.
Unfortunately, over time the characteristics of any display device can change. These changes can occur over a very short period of time (milliseconds) or over years. For example, when charge is stored at a pixel, the charge decays, affecting the brightness or color of the pixel. Alternatively, as time passes and a display device is used, the nature of the pixel can change: transistors become less efficient or responsive, impurities creep into display elements causing them to decrease in brightness or change in color, etc.
Referring to FIG. 7, a graph illustrating the typical light output of an OLED display as current is passed through the OLEDs is shown. The three curves represent typical performance of the different light emitters emitting differently colored light (e.g. R,G,B representing red, green and blue light emitters, respectively) as represented by luminance output over time or cumulative current. As can be seen by the curves, the decay in luminance between the differently colored light emitters can be different. The differences can be due to different aging characteristics of materials used in the differently colored light emitters, or due to different usages of the differently colored light emitters. Hence, in conventional use, with no aging correction, the display will become less bright and the color, in particular the white point, of the display will shift.
To some extent these changes can be ameliorated by modifying the operation of the device. For example, image information can be rewritten (refreshed) at each pixel site, operating voltages can be adjusted, more current can be made available, the timing of the control signals can be modified, data value to charge ratios can be changed, etc. In order to appropriately modify the operation of the device, however, the performance changes must be known.
One approach to compensating for uniformity and aging differences in a display is described in EP0923067 A1 by Kimura et al, and published 19990616. In this design a current measuring circuit is used to monitor the behavior of the display and the information used to modify the control of the display. In an alternative embodiment, a monitoring circuit is used to monitor the behavior of the display. In this design, complex current measuring circuits with comparators are necessary to provide useful information for modifying the control of the display.
U.S. Pat. No. 6,414,661 B1 issued Jul. 2, 2002 to Shen et al. describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light-emitting diodes (OLEDs) in an OLED display, by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. This technique requires the measurement and accumulation of drive current applied to each pixel, requiring a stored memory that must be continuously updated as the display is used, requiring complex and extensive circuitry.
U.S. Patent Application 2002/0167474 A1 by Everitt, published Nov. 14, 2002, describes a pulse width modulation driver for an OLED display. One embodiment of a video display comprises a voltage driver for providing a selected voltage to drive an organic light-emitting diode in a video display. The voltage driver may receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics. In one embodiment of the invention, the correction tables are calculated prior to and/or during normal circuit operation. Since the OLED output light level is assumed to be linear with respect to OLED current, the correction scheme is based on sending a known current through the OLED diode for a duration sufficiently long to allow the transients to settle out and then measuring the corresponding voltage with an analog-to-digital converter (A/D) residing on the column driver. A calibration current source and the A/D can be switched to any column through a switching matrix. This design requires the use of an integrated, calibrated current source and A/D converter, greatly increasing the complexity of the circuit design.
U.S. Pat. No. 6,504,565 B1 issued Jan. 7, 2003 to Narita et al., describes a light-emitting display which includes a light-emitting element array formed by arranging a plurality of light-emitting elements, a driving unit for driving the light-emitting element array to emit light from each of the light-emitting elements, a memory unit for storing the number of light emissions for each light-emitting element of the light-emitting element array, and a control unit for controlling the driving unit based on the information stored in the memory unit so that the amount of light emitted from each light-emitting element is held constant. An exposure display employing the light-emitting display, and an image forming apparatus employing the exposure display are also disclosed. This design requires the use of a calculation unit responsive to each signal sent to each pixel to record usage, greatly increasing the complexity of the circuit design.
JP 2002278514 A by Numeo Koji, published Sep. 27, 2002, describes a method in which a prescribed voltage is applied to organic EL elements by a current-measuring circuit and the current flows are measured; and a temperature measurement circuit estimates the temperature of the organic EL elements. A comparison is made with the voltage value applied to the elements, the flow of current values and the estimated temperature, the changes due to aging of similarly constituted elements determined beforehand, the changes due to aging in the current-luminance characteristics and the temperature at the time of the characteristics measurements for estimating the current-luminance characteristics of the elements. Then, the total sum of the amount of currents being supplied to the elements in the interval during which display data are displayed, is changed so as to obtain the luminance that is to be originally displayed, based on the estimated values of the current-luminance characteristics, the values of the current flowing in the elements, and the display data.
Published US Patent No. US20030122813 A1 entitled “Panel display driving display and driving method” by Ishizuki et al published 20030703 discloses a display panel driving device and driving method for providing high-quality images without irregular luminance even after long-time use. The value of the light-emission drive current flowing when causing each light-emission elements bearing each pixel to independently emit light in succession is measured, then the luminance is corrected for each input pixel data based on the above light-emission drive current values, associated with the pixels corresponding to the input pixel data. According to another aspect, the voltage value of the drive voltage is adjusted in such a manner that one value among each measured light-emission drive current value becomes equal to a predetermined reference current value. According to a further aspect, the current value is measured while an off-set current component corresponding to a leak current of the display panel is added to the current outputted from the drive voltage generator circuit and the resultant current is supplied to each of the pixel portions.
This design presumes an external current detection circuit sensitive enough to detect the relative current changes in a display due to a single pixel's power usage. Such circuits are difficult to design and expensive to build. Moreover, the measurement techniques are iterative and therefore slow and rely upon a voltage source drive while OLED displays are preferably controlled using constant current sources.
There is a need therefore for an improved aging compensation approach for organic light-emitting diode display.